Call site

In programming, a call site of a function (subroutine) is a line in the code which calls (or may call, through dynamic dispatch) a function. A call site passes zero or more arguments to the function, and receives zero or more return values.

Example

// this is a function definition
function sqr(x)
{
  return x * x;
}

// these are two call sites of the function
a = sqr(b);
c = sqr(b);

Assembler example

IBM/360 or Z/Architecture

* (usually) external call.... R13 usually points to a save area for general purpose registers beforehand
*                         and R1 points to a list of addresses of parameters (if any)
        LA    R1,=A(B)         point to (address of) variable 'B'
        L     R15,=A(SQR)      Load pointer (address constant) to separately compiled/assembled subroutine
        BALR  R14,R15          Go to subroutine, which returns - usually at zero displacement on R14
* internal call            (usually much smaller overhead and possibly 'known' parameters)
        BAL   R14,SQR          Go to program label and return

Sometimes, as an efficient method of indicating success or otherwise, return may be accomplished by returning at +0 or +4,+8 etc. requiring a small branch table at the return point - to go directly to process the case (as in HLL Switch statement).

        BAL   R14,SQR          Go to program label and return (using offset on R14 as return address)
        B     FAIL             (RET+0) - SOMETHING WRONG
*                              (RET+4) - O.K.

Conventionally however, a return code is set in R15 (0=OK, 4= failure, or similar ..) but requiring a separate instruction to test R15 or use directly as a branch index.

See also